Hydrogen-bonded 4H-1-benzopyrane-4-thione–water complexes properties in the S₂ excited state: the mechanism and dynamics of deactivation

Abstract

Unexpected results of spectral emission and photophysical study of S₂-excited 4H-1-benzopyrane-4-thione (BPT) in water solution were explained by intermolecular hydrogen bonding. There are at least two types of BPT–water complexes that participate in the S₂-fluorescence similarly as in S₀ → S₂ absorption. They both deactivate with high efficiency in internal conversion S₂ → S₁ and in intersystem crossing S₁ → T₁ processes.

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Introduction

Aromatic thioketones show many interesting spectral and photophysical properties including direct S₀ → T₁ absorption, thermally activated S₁-fluorescence, well-resolved S₀ → S₁, S₀ → S₂ and S₀ → S₃ absorption bands, fluorescence from the S₂ state, and phosphorescence from the T₁ state in solution at room temperature.^2–5 In perfluorohydrocarbons, the S₂(π,π*) state decay is exclusively intramolecular due to weak solute–solvent interactions. In all other solvents, irrespective of polarity and protic character, the S₂ state is known to be extremely short-lived because of efficient intermolecular quenching^2–8 or intracomplex deactivation as in the hydrogen-bonded thioketone–water complex.⁹ For illustration, for the xanthione whose S₂-state lifetime, τ_S2, is 152 ps in perfluoro-n-hexane, it is much shortened to τ_S2 = 38 ps in tetrachloromethane, τ_S2 = 24 ps in n-hexane, τ_S2 = 14 ps in acetonitrile and τ_S2 = 5 ps in toluene.^10,11 Similarly, for 4H-1-benzopyrane-4-thione, BPT (Fig. 1), the lifetime τ_S2 strongly depends on the solvent properties.¹² For thiocoumarin, TC, an isomer of BPT, a very short S₂-state lifetime, τ_S2 = 450 fs was found in n-hexane, and only 150 fs in acetonitrile.¹³ Irrespective of the S₂-state lifetime, the observed decays of fluorescence are almost always monoexponential.

Fig. 1 Structure of 4H-1-benzopyran-4-thione (BPT).

Unexpectedly, for a few aromatic thioketones and only in perfluorohydrocarbons and aliphatic hydrocarbons, the quantum yields of fluorescence have been determined, ϕ_F.^2,4,7,8,12 Moreover, no information has been published on the influence of the excitation wavelength, λ_exc, on the position and shape of their emission spectra and ϕ_F value. Because of a very short τ_S2, for TC, a strong dependence of the fluorescence spectrum position on λ_exc is observed. Upon TC excitation to the S₂ state with a large excess of vibration energy (Δν ∼ 5000 cm⁻¹) the fluorescence spectrum is strongly hypsochromically shifted.¹⁴ It is gradually shifted towards longer wavelengths upon TC excitation to the vibrational levels of the S₂ state with decreasing energy excess. For TC, in contrast to BPT, even in perfluoro-n-hexane, the S₂-state lifetime is very short, <1 ps, as indicated by a very small ϕ_F = 5 × 10⁻⁵.¹⁴ Most probably, the very fast process of conical intersection (intramolecular) is responsible for the very short τ_S2.

The lifetime in the S₂ state determined for BPT in water from emission decay as well as from the transient absorption decay, τ_S2, is just 1 ps, while in D₂O it is close to 2 ps.⁹ The twice longer τ_S2 in D₂O than in water points directly to a significant role of hydrogen bonds S⋯H(D)–O in deactivation of S₂-BPT in water.

The hydrogen bond and nonspecific interaction energies for the BPT–water complexes in the ground state S₀ and the excited singlet states S₁ and S₂ have been determined by us using ab initio computational methods.¹ In this paper we have shown that the BPT molecule can form four relatively strong hydrogen bonds with the water molecules in the ground S₀ as well as in the excited S₁ and S₂ electronic states. The energies of the hydrogen bonds formed between BPT and water molecules are predicted to significantly decrease in the S₁(n,π*) state and increase in the S₂(π,π*) state, when compared to that in the ground state S₀. It is worth noting that a similar effect has been found by Zhao and Han^15,16 for the BPT–methanol complex. The S₀ → S₁ and S₀ → S₂ transition energies calculated for the BPT–(H₂O)₄ complex, including the nonspecific solute–solvent interactions, are in very good agreement with the experimental absorption maxima of S₁ and S₂ states, determined for BPT in water.¹ Moreover, the changes in the hydrogen bond energies upon electronic excitation to the S₁ and S₂ states, determined from an experimental solvatochromic study, are in remarkably good agreement with those resulting from the ab initio calculation.¹

In the present paper a detailed S₂-emission spectral and photophysical study for BPT in water is presented. A computational study of difference in stoichiometry, structure and energy hydrogen-bonded BPT–water complexes in the electronic excited S₂ state and their influence on the S₂ → S₀ transition energies has been also performed. The results of these studies should provide information on the presence and deactivation mechanisms of intermolecular solute–solvent hydrogen-bonded complexes for BPT in their electronic excited S₂ state in water solution. They should also permit to conclude if the excess vibrational energy (in the range of 1000–5000 cm⁻¹) in the short-lived S₂ state influences the deactivation processes and the position and shape of the emission spectrum.

Computational and experimental methods

Experimental details

BPT was synthesized and purified by the methods described elsewhere.^17,18 Compounds 1-chloropropane (99%) (Aldrich), 1-chlorohexane (99%) (Aldrich), 1-chlorodecane (98%) (Aldrich), 1-chlorohexadecane (98%) (Fluka), and propionitrile (PrCN) (Fluka) were additionally dried over molecular sieves (A3, A4). Water was distilled and deionized. The absorption spectra were measured on a spectrophotometer Jasco V-650. Steady-state emission measurements were made on a Jobin Yvon-Spex Fluorolog 3-22 spectrofluorimeter. The emission spectra were corrected for the sensitivity of the detection system. Thioketone concentration was approximately 10⁻⁵–10⁻⁴ M. All measurements were performed at room-temperature. Fluorescence and phosphorescence quantum yield were measured by a relative method using quinine sulfate in 0.05 M H₂SO₄ (ϕ_F = 0.52)¹⁹ as a standard.

Because of a very low (<10⁻⁴) fluorescence quantum yield of BPT in the S₂ state, in water,⁹ to be sure that the weak emission, measured in the range of emission spectra and emission excitation spectra, comes exclusively from BPT and/or BPT complexes with water and not from the emitting impurities, the Ultra-High Performance Liquid Chromatography (UHPLC) was applied with the UV-VIS absorption and emission detection.²⁰ Irrespective of the wavelength of observation in the range of 250–390 nm (40 000–25 640 cm⁻¹), including all excitation wavelengths, λ_exc, the absorption chromatograms of BPT in acetonitrile shows only one peak. Similarly, the emission chromatograms measured for all λ_exc and many emission wavelengths, λ_em = 400–560 nm (25 000–17 860 cm⁻¹), from the range of S₂ emission spectra, there is only one peak of BPT of the same retention time as in the chromatogram of absorption. This type of chromatogram means that the sample does not contain absorbing and/or emitting impurities under the conditions of absorption measurements on a spectrophotometer and under the conditions of emission measurements on a spectrofluorimeter.

Calculation methods

All quantum-chemical calculations have been carried out using a Turbomole 6.6 quantum chemical software package.²¹ For all RI-DFT²² and RI-TDDFT^23,24 calculations the free-parameter PBE0 hybrid functional has been chosen.²⁵ This functional is constructed in such a way that a fixed fraction (1/4) of the Hartree–Fock exact exchange is added to the PBE exchange–correlation functional.²⁶ For structure preoptimization the double-ζ-quality SV(P) and corresponding auxiliary basis set have been employed.²⁷ Final structure optimization was carried out with a triple-ζ-quality basis set, TZVP²⁸ with analogous auxiliary basis set. The nature of all stationary points (the minima on the electronic potential energy surface) was confirmed by the frequency calculation on top of the optimized geometries. Due to the fact that in Turbomole package analytical second derivatives are not available for the electronic excited states, the excited state hessian matrix has been obtained by numerical differentiation of the excited state electronic energy gradients. All RI-DFT(PBE0) and RI-TDDFT(PBE0) calculations were carried out within the resolution-of-the-identity (RI) approximation which avoids direct evaluation of Coulomb four-center integrals. The structures obtained at the RI-DFT(PBE0)/TZVP and RI-TDDFT(PBE0)/TZVP levels of theory (respectively, for the ground and second excited state) have been subsequently used for the RICC2, CIS(D) and ADC(2)^29,30 single-point ab initio calculations. All ab initio calculations have been carried out with the ricc2 program²⁹ with correlation consistent, augmented triple-ζ-quality basis set.³¹ The Hartree–Fock reference wave functions were obtained within the RIJK approximation³² where direct evaluation of both Coulomb and exchange four-center integrals is avoided. Core orbitals have been frozen during all electronic-correlation ab initio calculations.

Results and discussion

Experimental study results

Spectral emission study in water. The steady-state emission spectra measurements for BPT in water were performed for a few excitation wavelengths, λ_exc, from all range of wavelengths covering the S₀ → S₂ band in the absorption spectrum. The energy of the S₃ state of BPT is by as much as ∼5000 cm⁻¹ higher than that of the S₂ state,³³ which permits a selective excitation to the S₂ state with different (even very large) excess vibrational energy. Selected spectra are presented in Fig. 2. All of them are characterized by the S₂-fluorescence band with the maximum at about 430–450 nm (22 220–23 260 cm⁻¹) and the T₁-phosphorescence band with the maximum at about 660 nm (15 150 cm⁻¹). It is easy to note that the shape as well as the maximum of the T₁-phosphorescence band does not depend on λ_exc, similarly as for BPT in perfluorohydrocarbons.^17,33 In contrast, the excitation wavelength value strongly influences the shape and the maximum of the S₂-fluorescence band. The decrease in the λ_exc value causes a widening of the S₂-fluorescence band and a bathochromic shift of this band maximum. In spite of λ_exc the Stokes shift, Δν^max_A–F, for BPT in water has smaller value than for BPT in non-specifically interacting solvents. Table 1 lists the S₂-fluorescence maxima (ν^max_F), FWHM (full width at half maximum) values of the S₂-fluorescence spectra (Δν_F^1/2) and the Stokes shifts, Δν^max_A–F = ν^max_A − ν^max_F, of BPT in water for different λ_exc. A comparison of the S₀ → S₂ absorption spectra and S₂-fluorescence spectra for BPT in water and in two non-specifically interacting solvents is presented in Fig. 3. The emission excitation spectra measured for BPT in water for different λ_em differ significantly from one another. These recorded at λ_em from the range of T₁-phosphorescence band are similar to one another and also similar to the S₀ → S₂ absorption spectrum. In contrast, these recorded at λ_em from the range of S₂-fluorescence band change their shape and intensity depending on λ_em and their maxima are shifted toward shorter wavelengths for longer λ_em (Fig. 4).

Fig. 2 Raw (a) and normalized (b) S₂-fluorescence and normalized (c) T₁-phosphorescence spectra of BPT in water.

Table 1 Dependence of spectral properties of the S₂-fluorescence band of BPT in water on the excitation wavelength, λ_exc, within the S₀ → S₂ absorption band

λ exc [nm/cm^−1]	ν ^maxF [cm^−1]	ΔνF^1/2 [cm^−1]	Δν^maxA–F [cm^−1]
ν ^maxA = 26 400 cm^−1 (from ref. 1)
330 (30 300)	21 920	5720	4480
340 (29 410)	21 920	5780	4480
350 (28 570)	22 000	5480	4400
360 (27 780)	22 460	5350	3940
376 (26 590)	23 040	4600	3360
396 (25 250)	23 150	4040	3250

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Fig. 3 Normalized S₀ → S₂ and S₀ → S_n, n ≥ 3, absorption and S₂-fluorescence spectra of BPT in selected non-specifically interacting solvents and in water.

Fig. 4 Normalized S₂-fluorescence (λ_em = 435 nm (22 990 cm⁻¹), 460 nm (21 740 cm⁻¹), 510 nm (19 610 cm⁻¹), 550 nm (18 180 cm⁻¹)) and T₁-phosphorescence (λ_em = 600 nm (16 670 cm⁻¹), 660 nm (15 150 cm⁻¹)) excitation spectra as well as S₀ → S₂ and S₀ → S_n, n ≥ 3, absorption spectra (I_abs) of BPT in water.

Photophysical emission study in water. The S₂-fluorescence quantum yield, ϕ_F, determined for BPT in water strongly depends on λ_exc and its value decreases with an increase in λ_exc. In contrast, the T₁-phosphorescence quantum yield, ϕ_P, only slightly depends on λ_exc. Taking into account the properties of BPT and xanthione^12,17 it can be assumed that under the conditions of measurements (constant concentration), the intensity of T₁-phosphorescence determine the relative contribution of T₁ state formation. Thus, the relative contribution of T₁-state formation in the S₂ excited state deactivation is the largest for shorter λ_exc. The S₂-fluorescence and T₁-phosphorescence quantum yields determined for different λ_exc are collected in Table 2.

Table 2 S₂-fluorescence, ϕ_F, and T₁-phosphorescence, ϕ_P, quantum yield of BPT in water

λ exc [nm/cm^−1]	ϕ F	ϕ F/ϕ^396F	ϕ P	ϕ P/ϕ^396P	(ϕF/ϕ^396F)/(ϕP/ϕ^396P)
ϕ ^396F, ϕ^396P – S2-fluorescence and T1-phosphorescence quantum yield determined for λexc = 396 nm (25 250 cm^−1).
330 (30 300)	0.00053	5.3	0.00083	1.4	3.8
340 (29 410)	0.00038	3.8	0.00084	1.4	2.7
350 (28 570)	0.00025	2.5	0.00079	1.4	1.8
360 (27 780)	0.00016	1.6	0.00073	1.3	1.2
376 (26 590)	0.00012	1.2	0.00066	1.1	1.1
396 (25 250)	0.00010	1.0	0.00058	1.0	1.0

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Spectral and photophysical emission study in non-specifically interacting solvent. In order to explain the reasons for the significant effect of λ_exc on the shape, position and quantum yield of S₂-fluorescence as well as the effect of λ_em on the shape of the S₂-fluorescence excitation spectra of BPT in water, we performed spectral and photophysical studies for BPT in solvents that are exclusively non-specifically interacting with this solute.¹ The steady-state emission spectra measurements were performed for BPT in the excited S₂-state in four 1-chloro-n-alkanes and propionitrile. These emission spectra were recorded at least for three different excitation wavelengths, λ_exc, from the long-wavelength band in the S₀ → S₂ absorption spectrum. For all non-specifically interacting solvents used, the excitation wavelength values do not influence the shape and the maximum, ν^max_F, of the S₂-fluorescence spectra. Moreover, the shape of these spectra is similar, irrespective of the solvent. The ν^max_F value shifts insignificantly bathochromically with increasing refractive index, n.³⁴ It means that in non-specifically interacting solvents, the dispersive solute–solvent interaction has a dominant effect on ν^max_F. The S₂-fluorescence excitation spectra measured for BPT in each solvent using different emission wavelengths, λ_em, do not differ significantly from each other and are similar to the S₀ → S₂ absorption spectrum. Table 3 lists the absorption and fluorescence maxima (ν^max_A and ν^max_F, respectively), FWHM values of the fluorescence spectra (Δν_F^1/2) and Stokes shifts, Δν^max_A–F = ν^max_A − ν^max_F, of BPT in all non-specifically interacting solvents used. It also gives some of solvent properties including the solvent refractive index, n, relative permittivity, ε, and polarity function, f(ε,n²), values. The S₂-fluorescence spectra of BPT in 1-chloro-n-hexadecane and propionitrile together with S₀ → S₂ absorption spectra are presented in Fig. 3, the S₂-fluorescence spectra of BPT in propionitrile recorded for different excitation wavelengths are in Fig. 5 and a comparison of fluorescence excitation spectra with absorption ones for BPT in 1-chloro-n-propane and propionitrile are in Fig. 6. The solvatochromic plot of the ν^max_F dependence on f(ε,n²) of BPT in non-specifically interacting solvents are given in Fig. 7.

Table 3 Spectral properties of the S₂-fluorescence band of BPT in non-specifically interacting solvents

Solvent	ν ^maxA ^a [cm^−1]	λ exc [nm/cm^−1]	ν ^maxF [cm^−1]	ΔνF^1/2 [cm^−1]	Δν^maxA–F [cm^−1]	f(ε,n^2)	ε	n
a From ref. 1, n – refraction coefficient; ε – relative permittivity; f(ε,n^2) = (ε − 1)/(2ε + 1) − (n^2 − 1)/(2n^2 + 1).
Propionitrile	27 030	372 (26 880)	22 300	4100	4730	0.292	28.26	1.363
1-Chloro-n-propane	27 100	344 (29 070)	22 030	4180	5070	0.226	8.59	1.388
		370 (27 030)	22 100	4220	5000
		400 (25 000)	22 030	4200	5070
1-Chloro-n-hexane	27 100	370 (27 030)	22 010	4250	5090	0.184	6.10	1.419
1-Chloro-n-decane	27 100	370 (27 030)	21 980	4220	5120	0.145	4.58	1.438
1-Chloro-n-hexadecane	27 100	370 (27 030)	21 920	4270	5180	0.110	3.70	1.450

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Fig. 5 Raw (a) and normalized (b) S₂-fluorescence spectra of BPT in propionitrile.

Fig. 6 Normalized absorption and S₂-fluorescence excitation spectra of BPT in selected non-specifically interacting solvents.

Fig. 7 Solvatochromic plot of the ν^max_F dependence on f(ε,n²) of BPT in selected solvents.

The S₂-fluorescence quantum yield values of BPT in 1-chloro-n-alkanes and propionitrile, in contrast to BPT in water, do not depend on the excitation wavelength and do not differ significantly from one another. The results are presented in Table 4.

Table 4 S₂-fluorescence quantum yield of BPT in non-specifically interacting solvents

Solvent	λ exc [nm/cm^−1]	ϕ F	ϕ F/ϕ^400F
ϕ ^400F – S2-fluorescence quantum yield determined for λexc = 400 nm (25 000 cm^−1).
Propionitrile	344 (29 070)	0.00071	0.9
	372 (26 880)	0.00071	0.9
	400 (25 000)	0.00083	1.0
1-Chloro-n-propane	370 (27 030)	0.00087
1-Chloro-n-hexane	370 (27 030)	0.00080
1-Chloro-n-decane	370 (27 030)	0.00074
1-Chloro-n-hexadecane	344 (29 070)	0.00073	0.9
	370 (27 030)	0.00074	0.9
	400 (25 000)	0.00082	1.0

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The similarity in shape of the S₂-fluorescence excitation spectra measured for BPT in non-specifically interacting solvents with the absorption S₀ → S₂ spectrum suggests that after excitation to the S₂ state with different excess vibration energy in the range of 0–5000 cm⁻¹, the ϕ_F value is constant. Slightly lower intensity of emission, I_F, than that of absorbed radiation, I_abs, for BPT in propionitrile (see Fig. 6) means – according to the relation I_F = ϕ_F·(10^−A(λ_exc)) – that the deactivation of BPT from higher excited states S_n, n ≥ 3, takes place partly with omission of the emitting S₂ state, similarly as for BPT in nonpolar perfluorohydrocarbons and hydrocarbons.

Computational study

As mentioned above, a detailed computational study of hydrogen-bonded BPT–water complexes in the ground S^rel₀ state and just after transition to the unrelaxed excited S^FC₁ and S^FC₂ states has been presented in ref. 1. The main aim of this study is the analysis of the presence of these complexes in the emitting S₂-excited state and their changes upon the S^rel₂ → S^FC₀ deactivation process. Therefore, the equilibrium structures of BPT molecules and different BPT–water complexes formed via hydrogen bonds were determined in their relaxed S₂-excited state (S^rel₂) using the RI-TDDFT(PBE0)/TZVP method. For these optimal geometries the energies of the hydrogen bonds in the S^rel₂ excited state and in the ground unrelaxed S^FC₀ state were predicted using the RICC2, ADC and CIS(D) methods. The calculations were performed for four different complexes of BPT molecules with a single water molecule, referred henceforth as A, B, C and D, and for the complexes formed by BPT molecules with two, three, four and five water molecules (see Fig. 8). The results presented in the current study as well as in ref. 1 clearly show that, despite the electronic state, the energy of hydrogen bonds formed by the sulphur atom of the thiocarbonyl group of the BPT molecule is significantly higher than that of the hydrogen bonds formed by the oxygen atom of BPT. Therefore, for the computational study and direct comparison with the experiment, we have chosen the complexes formed by the BPT molecule and two, three, four or five water molecules (see Fig. 8). The complex of BPT with two water molecules, dubbed here as AB, is stabilized by the creation of two strong hydrogen bonds between the water molecules and the sulphur atom of the thiocarbonyl group. The ABB′ complex, containing three water molecules, was created by adding the water molecule on the optima location of the C complex to the AB complex. In the course of the geometry optimization the ABB′ molecular system was obtained where three water molecules are located in the surrounding of the sulphur atom. The ABD complex was formed in an analogous way, by supplementing the AB system with additional water molecules placed in the optimal location of the D complex. The ABCD complex contains a BPT molecule and four water molecules. Two water molecules form, in full analogy with the AB complex, the hydrogen bonds with a sulphur atom. The remaining two water molecules are located in the surrounding of the oxygen atom. The largest molecular system considered in the current study, dubbed here as ABB′CD, is formed by a BPT molecule and five water molecules. This system been created by extending the ABCD complex with additional water molecules placed in the neighbourhood of one of the carbon atoms. Further extension of the systems with water molecules has not been reflected in the increase of complex stability. Therefore in the current study we have not considered complexes with more than five water molecules. The geometries of all mentioned molecular complexes were relaxed in their S₀ and S₂ electronic states.

Fig. 8 Equilibrium structures of BPT–water complexes in the excited S^rel₂ state.

Table 5 gives the predicted hydrogen bond energies, E_HB, in the ground states S^rel₀ and S^FC₀ and in the excited states S^FC₂ and S^rel₂ of the BPT–water complexes. It is worth mentioning that the energies determined in S^rel₀ and S^FC₂ taking part in the process of absorption, for A, B, C, D and AB complexes, are in qualitative agreement with the values determined in ref. 1. Very important to notice from Table 5 is that the hydrogen bond stabilization energy for all studied complexes formed by the BPT molecule with more than one water molecule (AB, ABD, ABB′ and especially ABCD and ABB′CD) is very high irrespective of the electronic state. For all BPT–water complexes considered, the total hydrogen bond energy increases as a result of S^rel₀ → S^FC₂ excitation, irrespective of the calculation method. As can be noted from Table 5, the hydrogen bond energy changes upon the S^rel₂ → S^FC₀ deactivation process are generally small and their sign depends on the calculation method.

Table 5 Hydrogen bond energies (in cm⁻¹) calculated using RICC2, ADC(2) and CIS(D) methods of the isolated BPT–water complexes in their ground and second excited singlet electronic states obtained at the optimal TDDFT(PBE0)/TZVP geometries

Complex	Optimal geometry	Electronic state	E HB
			RICC2	ADC(2)	CIS(D)
A	S0	S^rel0	2120	1980	1980
		S ^FC 2	2337	2252	2412
	S2	S ^rel 2	2306	2091	2234
		S^FC0	2135	2047	2047
B	S0	S^rel0	2578	2449	2449
		S ^FC 2	2815	2741	2949
	S2	S ^rel 2	3066	2781	2954
		S^FC0	2902	2844	2844
C	S0	S^rel0	1256	1210	1210
		S ^FC 2	1202	1172	1185
	S2	S ^rel 2	1471	1391	1450
		S^FC0	1262	1227	1227
D	S0	S^rel0	1326	1298	1298
		S ^FC 2	1072	1020	957
	S2	S ^rel 2	982	988	937
		S^FC0	1214	1169	1169
AB	S0	S^rel0	4657	4407	4407
		S ^FC 2	5076	4894	5253
	S2	S ^rel 2	5242	4666	4946
		S^FC0	5011	4911	4911
ABB′	S0	S^rel0	6182	5892	5892
		S ^FC 2	6705	6480	6944
	S2	S ^rel 2	6928	6153	6501
		S^FC0	6658	6577	6577
ABD	S0	S^rel0	5904	5620	5620
		S ^FC 2	6202	5977	6268
	S2	S ^rel 2	6342	5765	6002
		S^FC0	6190	6044	6044
ABCD	S0	S^rel0	7214	6880	6880
		S ^FC 2	7427	7187	7535
	S2	S ^rel 2	7765	7130	7421
		S^FC0	7555	7352	7352
ABB′CD	S0	S^rel0	8687	8313	8313
		S ^FC 2	9025	8746	9193
	S2	S ^rel 2	9369	8569	8928
		S^FC0	9123	8932	8932

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To make the calculation conditions closer to the experimental ones, the macroscopic solvent effect was investigated using the COSMO solvation model as implemented in the Turbomole package.²¹ The predicted energies of nonspecific interactions of BPT–water complexes in S^rel₀, S^FC₂, S^rel₂ and S^FC₀ states with bulk water molecules are collected in Table 6. As can be seen from these results, the highest stabilization energy due to the nonspecific interactions is observed for ABB′CD and ABCD complexes. In contrast to the results obtained in ref. 1 using the conductor polarisable continuum model (CPCM), no substantial differences in the energies of nonspecific interactions between electronic states were observed. On the basis of the predicted high energies of hydrogen bonds (Table 5) and nonspecific interactions (Table 6), it can be concluded that practically all BPT molecules in S^rel₀, S^FC₂, S^rel₂ and S^FC₀ states in the water solution formed complexes with five or four water molecules via hydrogen bonds. The highest energies of hydrogen bonds and nonspecific interactions determined for ABB′CD complexes suggest that the concentration of complexes with five water molecules in the surrounding of the BPT molecule is definitely the highest before excitation in S^rel₀ as well as in the emitting S^rel₂ state.

Table 6 Impact of the nonspecific interactions on the binding energies (in cm⁻¹) of BPT–water complexes in their ground and second excited singlet electronic states

Complex	S^rel0 ^a	S^FC2 ^a	S^rel2 ^b	S^FC0 ^b
a Calculated for the equilibrium structure of the ground state S0. b Calculated for the equilibrium structure of the excited state S2.
A	1293	1391	1380	1232
B	1044	1052	758	783
C	1658	1649	1500	1649
D	1791	1920	1965	1817
AB	2381	2474	2190	2061
ABB′	3080	3200	2849	2706
ABD	4184	4293	4131	3887
ABCD	5002	5160	4721	4644
ABB′CD	5651	5820	5367	5276

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Further theoretical investigation as well as an in-detail discussion of all relevant issues is the subject of an upcoming article.

Discussion

The above-presented results of spectral emission and photophysical study for BPT in water clearly show that the shape and maximum of the S₂-fluorescence spectra as well as S₂-fluorescence quantum yield strongly depend on λ_exc. Moreover, the shape of S₂-fluorescence excitation spectra depends on λ_em. The most probable explanation of such results is the presence of at least two different species in the S^rel₂ emitting state. As mentioned above, on the basis of computational study results, practically all BPT molecules in water solution form with water molecule hydrogen-bonded complexes in the electronic states S^rel₀, S^FC₂, S^rel₂ and S^FC₀. The calculated energies of the hydrogen bond and nonspecific interaction clearly show that the most stable, irrespective of the electronic state, should be the complexes formed via hydrogen bonds by the BPT molecule with five water molecules (which occur in majority) or with four water molecules. Therefore, we propose that there must be at least two different types of hydrogen-bonded complexes (ABB′CD, referred henceforth as K1 and ABCD, referred henceforth as K2), which are the species responsible for such unexpected S₂-fluorescence. We tried to show that the detailed analysis of the experimental study results confirmed this assumption. It is worth noting that the source of fluorescence cannot be the impurities, as explained in the section Experimental details.

Absorption spectra of BPT–water hydrogen-bonded complexes. It is easy to notice from Table 5 and ref. 1 that the hydrogen bond energy changes due to S^rel₀ → S^FC₂ excitation are not great and they do not differ significantly for BPT–water complexes with five and four water molecules. Therefore, the S₀ → S₂ absorption spectrum measured for BPT in water, which is the sum of absorption spectra of two types of BPT–water hydrogen-bonded complexes K1 and K2, is not shifted significantly with respect to the spectra of BPT in non-specifically interacting solvents.¹ The molar absorptivity, ,^35,36 determined for BPT in water is significantly higher than for BPT non-specifically interacting solvents.^4,12

S₂-fluorescence spectra and quantum yield of BPT–water hydrogen-bonded complexes. Similar to the absorption spectra, the S₂-fluorescence spectra measured for BPT in water are the sum of the spectra of two types of BPT–water hydrogen-bonded complexes K1 and K2. Because of the differences in the shape and position of these spectra, the contribution of K1 and K2 must depend on the λ_em value. The S₂-fluorescence spectra with the maximum, ν^max_F = 21 920 cm⁻¹ (456 nm) recorded for the shorter excitation wavelength, i.e. λ_exc = 330–350 nm (30 300–28 570 cm⁻¹, see Fig. 2), are similar in shape and are characteristic of complex K1. The S₂-fluorescence spectra with the maximum, ν^max_F = 23 150 cm⁻¹ (432 nm) recorded for the longest excitation wavelength, i.e. λ_exc = 396 nm, are characterized for relatively the largest contribution of complex K2.

Assuming that the S₂-fluorescence spectrum recorded for λ_exc = 340 nm (29 410 cm⁻¹) is the most characteristic of complex K1, we determined the spectrum of complex K2 as the difference between the spectrum recorded for λ_exc = 396 nm (25 250 cm⁻¹) (at which the contribution of K2 is the highest) and that recorded for λ_exc = 340 nm (29 410 cm⁻¹). The results are presented in Fig. 9. Than from the spectrum recorded for λ_exc = 360 nm (27 780 cm⁻¹), which is the sum of different contributions of K1 and K2 spectra, we subtracted the spectrum of complex K2 (ν^max_F = 23 740 cm⁻¹), determining the spectrum of complex K1. It is easy to notice that the spectrum of complex K1 is similar to that recorded for λ_exc = 340 nm (29 410 cm⁻¹) (and also λ_exc = 330 nm (30 300 cm⁻¹) and 350 nm (28 570 cm⁻¹)), which confirmed our above-mentioned assumption. As the S₂-fluorescence spectra recorded for BPT in water for λ_exc = 330–350 nm (30 300–28 570 cm⁻¹) are similar, no effect of λ_exc on the position and shape of S₂-fluorescence spectrum of complex K1 is observed, in spite of significant excess of vibrational energy (3000–5000 cm⁻¹) in the short-lived S₂ state (τ_S2 ≤ 1 ps),⁹ in contrast to the situation for S₂-thiocoumarin in perfluorohydrocarbon and hydrocarbon solvents (τ_S2 = 0.45 ps in n-hexane).^13,14 It implies that the process of redistribution of vibrational energy in complex K1 and the solvent cooling of this complex are very fast in water, much faster than for S₂-thiocoumarin in n-hexane.¹⁴ It is worth noting that the solvent cooling process was found to be very fast also for N-acetyltryptophanamide in water³⁷ and for hot purine derivatives in heavy water.³⁸ The stable shape and position of ν^max_F in the experimental S₂-fluorescence spectra recorded for λ_exc = 330–350 nm (30 300–28 570 cm⁻¹), but the quantum yield of S₂-fluorescence, ϕ_F, varying with λ_exc, indicate the dependence of ϕ_F of complex K1 on λ_exc. The significant increase in the width of the band Δν_F^1/2, in the experimental S₂-fluorescence of BPT in water with increasing excitation energy (decreasing λ_exc, λ_exc ≤ 350 nm (28 570 cm⁻¹), see Table 1), can be explained by the occurrence of emission from the vibrationally excited S₂ state (ν > 0) with a higher quantum yield than from the S^rel₂ state of complex K1. An indirect confirmation of this interpretation is the lack of influence of λ_exc on the shape of S₂-fluorescence spectra (see Table 3) and ϕ_F value (see Table 4) for BPT in non-specifically interacting solvents.

Fig. 9 Normalized S₂-fluorescence spectra of BPT–water complexes K1 and K2 in water solution.

In order to estimate the contribution of complexes K1 and K2 to the S₂-fluorescence as a function of λ_exc and λ_em, the relative quantum yields were calculated, defined as ϕ_F(rel) = I_F/I_abs, where I_F – S₂ is the fluorescence intensity for specific λ_exc and λ_em, I_abs is the intensity of absorbed radiation determined for λ_exc, I_abs = 1–10^−A(λ_exc), where A is the absorbance. The correlations between the values of ϕ_F(rel)³⁰⁰ determined for λ_exc = 330 nm (30 300 cm⁻¹) (when practically only complex K1 contributes to S₂-fluorescence) and the values of ϕ_F(rel)³⁹⁶ determined at λ_exc = 396 nm (25 250 cm⁻¹) (at which the relative contribution of complex K2 to S₂-fluorescence is the highest), for selected λ_em are given in Table 7. The values of λ_em were chosen to correspond to λ_em values at which the lifetimes of S₂-fluorescence⁹ and emission excitation spectra were measured. It can be noticed from Table 7 that the highest ratio of ϕ_F(rel)³³⁰/ϕ_F(rel)³⁹⁶ ≈ 8 was determined for λ_em = 510–530 nm (19 610–18 870 cm⁻¹), at which almost exclusively the emission from complex K1 is observed (see Fig. 9). This value is much higher than ϕ³³⁰_F/ϕ³⁹⁶_F = 5.3 obtained for the quantum yields determined on the basis of the areas of the measured S₂-fluorescence spectrum which is the sum of the spectra of complexes K1 and K2 (see Table 2). For λ_em = 410–435 nm (24 390–22 990 cm⁻¹), so in the range in which the contribution of complex K2 to the emission is relatively the greatest (see Fig. 9), the ratio of ϕ_F(rel)³³⁰/ϕ_F(rel)³⁹⁶ ≈ 3 was determined as significantly smaller than ϕ³³⁰_F/ϕ³⁹⁶_F = 5.3. These results confirm the assumption that the experimentally recorded S₂-fluorescence spectra must be a sum of the spectra of at least two species. Moreover, the contribution of complex K2 characterised by lower quantum yield is greater in the short-wavelength range 400–450 nm (25 000–22 220 cm⁻¹), while that of complex K1 characterised by higher quantum yield is greater in the long-wavelength range 500–550 nm (20 000–18 180 cm⁻¹). It should be also noted that in the range of emission wavelengths corresponding to T₁-phosphorescence, i.e. λ_em > 600 nm (16 670 cm⁻¹), the ratio of ϕ_P(rel)³³⁰/ϕ_P(rel)³⁹⁶ (where ϕ_P(rel) = I_P/I_abs and I_P is T₁-phosphorescence intensity) is constant and corresponds to ϕ³³⁰_P/ϕ³⁹⁶_P = 1.4 obtained for the quantum yields estimated on the basis of the areas measured under the measured T₁-phosphorescence spectrum.

Table 7 S₂-fluorescence intensity, I_F, T₁-phosphorescence intensity, I_P, and relative quantum yield ϕ_F(rel) and ϕ_P(rel), for λ_exc = 330 nm (30 300 cm⁻¹) and 396 nm (25 250 cm⁻¹) and for selected values of λ_em from the range of S₂-fluorescence and T₁-phosphorescence, for BPT in water

λ em [nm/cm^−1]	I ^330F/I^396F	ϕ F(rel)^330/ϕF(rel)^396
I ^330F, I^396F – S2-fluorescence intensity for λexc = 330 and 396 nm; I^330P, I^396P – T1-phosphorescence intensity for λexc = 330 and 396 nm; ϕF(rel)^330, ϕF(rel)^396, ϕP(rel)^330, ϕP(rel)^396 – relative S2-fluorescence and T1-phosphorescence quantum yield determined for λexc = 330 and 396 nm, respectively.
410 (24 390)	0.86	3.52
425 (23 530)	0.75	3.06
435 (22 990)	0.89	3.65
460 (21 740)	1.18	4.84
480 (20 830)	1.47	6.02
510 (19 610)	2.00	8.20
520 (19 230)	1.97	8.07
530 (18 870)	1.82	7.43

---

λ em [nm/cm^−1]	I ^330P/I^396P	ϕ P(rel)^330/ϕP(rel)^396
600 (16 670)	0.44	1.80
620 (16 130)	0.33	1.34
640 (15 620)	0.38	1.35
660 (15 150)	0.31	1.25

---

Emission excitation spectra of BPT–water hydrogen-bonded complexes. As mentioned above, the S₂-fluorescence excitation spectra measured for BPT in water with different λ_em differ significantly. But it can be noticed from Fig. 4 that the spectra recorded for λ_em = 510 nm (19 610 cm⁻¹) and 550 nm (18 180 cm⁻¹) are similar and their long-wavelength maximum is shifted hypsochromically with respect to the spectra recorded for other λ_em and the experimental absorption spectrum of BPT in water. Taking into regard these results, S₂-fluorescence spectra determined for complexes K1 and K2 (Fig. 9) and the fact that the highest ratio of ϕ_F(rel)³³⁰/ϕ_F(rel)³⁹⁶ was determined for λ_em = 510–530 nm (19 610–18 870 cm⁻¹), it can be assumed that the S₂-fluorescence excitation spectrum recorded for λ_em = 510 nm (19 610 cm⁻¹) is characteristic of complex K1 exclusively. A significant difference in the shape and intensity of this S₂-fluorescence excitation spectrum and the absorption spectrum is a result of a significant dependence of ϕ_F of complex K1 on λ_exc. As follows from Fig. 4 after the normalization of S₂-fluorescence excitation spectra of BPT in water at the λ_exc = 396 nm (25 250 cm⁻¹), the emission intensity at λ_exc = 330 nm (30 300 cm⁻¹) is the highest for λ_em = 510 nm (19 610 cm⁻¹) and 550 nm (18 180 cm⁻¹) and decreases gradually for λ_em = 460 nm (21 740 cm⁻¹) and 435 nm (22 990 cm⁻¹). The ratio of emission intensity determined at λ_exc = 330 nm (30 300 cm⁻¹) on the basis of these data for selected λ_em is equal to I⁵¹⁰_F/I⁴³⁵_F = 2.0 and I⁴⁶⁰_F/I⁴³⁵_F = 1.6. These values are consistent with the ratio of (ϕ_F(rel)³³⁰/ϕ_F(rel)³⁹⁶) = X dependence on the λ_em determined on the basis of S₂-fluorescence spectra (see Table 7), where X⁵¹⁰/X⁴³⁵ = 2.2 and X⁴⁶⁰/X⁴³⁵ = 1.7. These data confirm that ϕ_F of complex K1 is higher than for complex K2 and it depends on the λ_exc value.

The shape and intensity of the emission excitation spectra measured for BPT in water, for λ_em from the range of T₁-phosphorescence spectrum, do not depend on the λ_em value and are similar to the S₀ → S₂ absorption spectrum in the wide range of wavelength 260–410 nm (38 460–24 390 cm⁻¹). It implies that both complexes K1 and K2 deactivate to the T₁ state in processes S₂ → S₁ → T₁, which differ from the situation for BPT molecules itself (in perfluorohydrocarbons), which deactivate from the S₂ state not only to the S₁ state, but also directly to S₀ with a contribution of ϕ_S2S0 ∼ 0.10.³³

S₂-fluorescence lifetime of BPT–water hydrogen-bonded complexes. The very short lifetime, τ_S2 = 1.0 ± 0.1 ps, of BPT–water complexes in the S₂ state was determined by means of the fluorescence up-conversion technique and by transient absorption spectroscopy.⁹ The fluorescence measurements were performed for 350 nm (28 570 cm⁻¹) and 380 nm (26 310 cm⁻¹) excitation wavelength and the analysed fluorescence wavelength from 425 nm (23 530 cm⁻¹) to 550 nm (18 180 cm⁻¹), while the fitting analysis was carried out using the single exponential function. It is easy to notice in Fig. 3 in ref. 9 that the τ_S2 value determined on the basis of the decay obtained upon 380 nm (26 310 cm⁻¹) photoexcitation depends on the fluorescence wavelength. Both for BPT in water and in D₂O (τ_S2 = 2.10 ± 0.15 ps) the τ_S2 value is the longest for the fluorescence wavelength 425 nm (23 530 cm⁻¹) and the shortest for 550 nm (18 180 cm⁻¹). Moreover, the τ_S2 value determined on the basis of the fluorescence decay for BPT in water, probed at 490 nm (20 410 cm⁻¹) following 350 nm (28 570 cm⁻¹) photoexcitation, is shorter than following the 380 nm (26 310 cm⁻¹) one. A more extensive analysis³⁹ allows noting that all experimental fluorescence decay curves contain an additional ultrafast component centred at around 750 fs (see Fig. 3 in ref. 9). By a process of elimination, the ultrafast signal was assigned to the BPT–water intermolecular complex quenching process, while the signal observed on the longer time-scale is ascribed to the decay of the S₂-BPT molecules.³⁹ The results presented in this study, both experimental and computation ones, allow us to verify that hypothesis. As mentioned above, practically all BPT molecules form in water solution BPT–water complexes in S₀ and S₂ states. Therefore, the S₂-fluorescence decay in water must describe the decay of two types of BPT–water complexes K1 and K2, and not BPT molecules. Their participation in the decay depends on photoexcitation, λ_exc and fluorescence wavelengths, λ_em. Taking into regard the S₂-fluorescence spectra of complexes K1 and K2 (Fig. 9) and practically the exclusive contribution of complex K1 in the emission for λ_em = 520 nm (19 230 cm⁻¹) and 550 nm (18 180 cm⁻¹), it can be assumed that the S₂-fluorescence decay recorded for λ_exc = 380 nm (26 310 cm⁻¹) and the above λ_em describe the decay of complex K1. The values of τ_S2 = 0.97–1.02 ps determined in water solution and τ_S2 = 1.99–2.06 ps in D₂O under these measurement conditions are characteristic of complex K1. Unfortunately, because of a higher concentration of complex K1 than K2, both in the ground S₀ as well as the excited S₂ state, it is impossible to select such conditions of measurements to make sure that S₂-fluorescence would come exclusively from complex K2. The relatively greatest contribution of this complex in S₂-fluorescence decays is when this decay is measured following 380 nm (26 310 cm⁻¹) photoexcitation and λ_em = 425 nm (23 530 cm⁻¹). Because of the longest τ_S2 determined under such conditions in both water and D₂O solutions, it can be assumed that τ_S2 of complex K2 must be longer than that of K1, and its value should be longer than 1.03 ps in water solution and longer than 2.20 ps in D₂O (see Fig. 3 in ref. 9). The above reasoning is confirmed by a shorter τ_S2 value determined for the fluorescence wavelength 490 nm (20 410 cm⁻¹) following 350 nm (28 570 cm⁻¹) excitation (when the relative contribution of K1 into S₂-fluorescence spectra is greater) than 380 nm (26 310 cm⁻¹) photoexcitation.⁹ Moreover, the transient absorption measurements with photoexcitation at 380 nm give shorter time constant, τ = 0.9 ps, with analysing wavelength at 520 nm (19 230 cm⁻¹) than at 425 nm (23 530 cm⁻¹), when τ = 1.1 ps.³⁹ The above-mentioned ultrafast component (750 fs) observed as a clear unfit in almost all experimental fluorescence decay curves (see Fig. 3 in ref. 9) cannot come from the emission of complexes K1 and K2, its contribution in the total decay shows a distinctly different dependence on λ_exc and λ_em than the decay of these two complexes. This component can come from the processes of vibrational energy redistribution and solvent relaxation. This ultrafast component is responsible for some shortening in τ_S2 determined for complexes K1 and K2. However, quantitative determination of this effect is impossible.

S₂-excited state deactivation processes of BPT–water hydrogen-bonded complexes. On the basis of quantum yields, ϕ_F, the S₂-fluorescence determined for BPT in water in this study and the results of τ_S2 measurements⁹ presented above, the radiation rate constant, k_F = ϕ_F/τ_S2, can be found for BPT–water complexes K1 and K2. For complex K2, τ_S2 ≥ 1.03 ps and ϕ_F ∼ 1 × 10⁻⁴, therefore k_F ≤ 1 × 10⁸ s⁻¹. Taking into consideration the τ_S2 = 0.99 ps measured at λ_exc = 350 nm (28 570 cm⁻¹) and λ_em = 490 nm (20 410 cm⁻¹) (when the emission of complex K1 is much larger than that of complex K2) and ϕ_F = 2.5 × 10⁻⁴ determined for λ_exc = 350 nm (28 570 cm⁻¹), and k_F = 2.5 × 10⁸ s⁻¹ for complex K1.

As mentioned above, in water solution in the emitting S^rel₂ state, BPT does not occur in the form of molecules, but exclusively as BPT–water complexes. The energy gap ΔE(S^rel₂ − S^FC₁) determined in this study on the basis of theoretical calculations for BPT complexes with five (ABB′CD–K1) and with four (ABCD–K2) water molecules are 4800 and 5400 cm⁻¹ (calculated using the ADC(2) method). These values are by over 3000 cm⁻¹ smaller than ΔE(S^rel₂ − S^FC₁) for BPT in perfluorohydrocarbons.^4,33 The reason for such a great decrease in the S₂ − S₁ energy gap is a significant weakening of the energy of hydrogen bonds type A and B (see Fig. 8) formed by the sulfur atom in the thiocarbonyl group of BPT and water molecules in the S₁(n,π*) state.¹ Because of experimentally determined low values of quantum yields of S₂-fluorescence and S₂-phosphorescence processes for BPT in water (ϕ_F = 1–5 × 10⁻⁴ and ϕ_P = 6–8 × 10⁻⁴, respectively), no photochemical changes even upon long irradiation of BPT in water (ϕ_PCH < 10⁻³), and a decrease in the S₂ − S₁ energy gap, the dominant process in S^rel₂ deactivation of complexes K1 and K2 must be the internal conversion to the S₁ state. A similar value of S₂ − S₁ energy gap for both complexes is reflected in a similar value of deactivation constant of S₂ → S₁, and thus a similar value of their τ_S2.⁹

According to the energy gap law for rigid aromatic thioketones in perfluorohydrocarbons, a linear dependence between log k_nr and ΔE(S^rel₂ − S^FC₁) was obtained⁵ and is presented in Fig. 10. On the basis of this relation, it is possible to find the values of k_S2S1 ≈ k_nr corresponding to the values of ΔE(S^rel₂ − S^FC₁) calculated for complexes K1 and K2. These values are 5.6 × 10¹⁰ s⁻¹ for complex K1 and 3.8 × 10¹⁰ s⁻¹ for complex K2, so τ_S2 ∼ 1/k_S2S1 should be of 17.9 ps and 26.3 ps, respectively. However, the value experimentally determined for BPT in water is τ_S2 ∼ 1 ps. So, the electronic coupling matrix elements between S₂ and S₁ states in BPT–water complexes must be 5–6 times greater than for S₂ and S₁ states of the BPT molecule. The lifetime of S₁(n,π*) is τ_S1 ≪ 1 ps,⁹ while the radiation rate constant is k_F < 10⁵ s⁻¹, so the quantum yield of S₁-fluorescence is very small, of ϕ_F ∼ 10⁻⁷. This explains why the S₁ state emission is not visible. The S₁ state has not been seen in the transient absorption spectra measured for different delay times for BPT in water.³⁹

Fig. 10 Plot of the logarithm of the rate constant for nonradiative decay processes of the S₂ state, k_nr, vs. S^rel₂ − S^FC₁ energy gap, ΔE(S₂ − S₁), for rigid thioketones in perfluorohydrocarbon solution at room temperature (from ref. 5) together with BPT–water hydrogen bonded complexes with five or four water molecules (K1 = ABB′CD, K2 = ABCD).

As shown above, the phosphorescence spectra and quantum yield, ϕ_P, do not depend significantly on the excitation wavelength. Moreover, the shape of the phosphorescence excitation spectra does not depend on λ_em and is similar to that of the absorption intensity spectrum. It means that both types of S₁-excited BPT–water complexes (K1 and K2) deactivate to the T₁ state with the large and similar yield.

Properties of S₂-excited BPT–water hydrogen-bonded complexes compared to those of S₂-BPT. BPT in water solution, both in the ground state S₀ and in the excited state S₂ occurs exclusively in the form of BPT–water complexes formed by strong hydrogen bonds, while in the solvents interacting only non-specifically with it (perfluorohydrocarbons, hydrocarbons, 1-chloro-n-alkanes, and propionitrile) BPT occurs only in the form of molecules.

Because of small changes in the energy of hydrogen bonds between BPT and water, as a result of electronic transitions S^rel₀ → S^FC₂S^FC₂ and S^rel₂ → S^FC₀, the positions of ν^max_A and ν^max_F for BPT–water complexes and for the BPT molecule in solvents of similar polarity and polarizability, interacting only non-specifically with it, e.g. propionitrile, do not differ significantly.

BPT–water complexes in water solution show much higher than BPT molecules in solvents with which it interacts only non-specifically, = 16 000–17 000 M⁻¹ cm⁻¹.^4,40 Similarly for BPT–water complexes K1 in water solution the values of k_F are significantly higher than for BPT molecules in the exclusively non-specifically interacting solvents.^4,35

It should be emphasised that much (∼25 times) higher k_S2S1 ≈ k_nr for deactivation of the S^rel₂ state of BPT–water complexes than for BPT molecules^4,5 implies that the electron coupling between S₂(π,π*) and S₁(n,π*) states in BPT–water complexes is about ∼5 times stronger than in the BPT molecule.

Conclusions

The results of the computational study clearly show that in water solution the BPT molecule in its ground S₀ and excited S₂ electronic states can form solute–solvent complexes via strong hydrogen bonds. As follows from the high values of the hydrogen bond energies, it should be assumed that almost all BPT molecules in the emitting S^rel₂ state form complexes with four or three water molecules. Therefore, the S₂-fluorescence is connected exclusively with BPT–water S₂-excited complexes and not with S₂-excited BPT molecules, as it was assumed in ref. 9. Detailed spectral and photophysical emission study shows that at least two different types of BPT–water complexes must exist in the S^rel₂ state and participate in S₂-fluorescence process. One of them, with significantly higher concentration in solution, is characterized by longer wavelength S₂-fluorescence spectra and higher S₂-fluorescence quantum yield which depends on the excitation wavelength value. The hydrogen bonds formed between the BPT molecules and water molecules play a dominant role in the processes of radiationless deactivation from the S₂ state. Much lower energy of these bonds in the S₁(n,π*) state than in the S₂(π,π*) state, similarly as observed for BPT in methanol,¹⁵ means that the S₂ − S₁ energy gap is much smaller. Moreover the electron coupling between these states in the complexes is much greater than in the BPT molecule. As a consequence, both types of S₂-excited BPT–water complexes deactivate in very short time (∼1 ps) in a very fast S₂ → S₁ internal conversion process. Correct interpretation of the BPT properties in water solution in the excited S^rel₂ state was possible thanks to the analysis of the spectral and photophysical experimental results and on the basis of advanced theoretical calculations of hydrogen bond energies for BPT–water complexes in S^rel₂, S^FC₁ and S^FC₀ states. Similarly as for BPT in water the dual fluorescence of two hydrogen-bonded methyl salicylate–water complexes was recently observed.⁴¹

Acknowledgements

We thank Julia Józkowiak for technical support and Maria Spychalska for her help in manuscript preparation. Calculations were performed at the Poznań Supercomputer Centre PCSS.

Notes and references

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